1. Field of the Invention
The invention relates to a crystalline silicon thin film semiconductor device, a crystalline silicon thin film photovoltaic device, and a process for producing a crystalline silicon thin film semiconductor device, and more particularly to a crystalline silicon thin film semiconductor device, a crystalline silicon thin film photovoltaic device, and a process for producing a crystalline silicon thin film semiconductor device, wherein a polycrystalline silicon thin film is formed using amorphous silicon as a seed crystal.
2. Prior Art
In semiconductor devices, such as solar cells, in order to form a high-quality crystalline silicon device having a thickness of about 1 to 4 xcexcm on a glass substrate having thereon a conductive film (layer), a high-quality seed crystal should be formed directly on the glass substrate having thereon a conductive film. Requirements to be satisfied in the formation of this seed crystal include:
(1) high crystallinity (high degree of crystallization);
(2) crystallographic orientation;
(3) large throughput at a time; and
(4) low-temperature process which permits the use of general glass substrates.
In the production of solar cells, a production process has hitherto been adopted wherein a polycrystalline silicon thin film is formed on a dissimilar substrate, such as glass. According to this production process, there is no need to use large-area and high-quality silicon crystal substrates, and, thus, a significant reduction in cost can be expected. In the production of semiconductor devices having good characteristics, however, the quality of polycrystalline silicon thin films should be improved. To this end, in general, quartz or the like, which can withstand high temperature, is used as the substrate, and this substrate is subjected to high-temperature deposition treatment to form a silicon thin film having good crystallinity. In this production process, however, since expensive substrates, such as quartz, are used, a reduction in cost cannot be realized.
In order to solve this problem, a method has been proposed in K. Yamomoto et al., IEEE xe2x80x9cFirst World Conference on Photovoltaic Energy Conversion,xe2x80x9d 1575-1578 (1994). According to this method, amorphous thin film silicon is melted and crystallized, for example, by laser annealing to form a film on the surface of a substrate, thereby producing polycrystalline thin film silicon having good crystallinity. This method is advantageous in that, since the temperature rise of the substrate can be suppressed, the use of low-cost substrate materials becomes possible. Further, an attempt to form polycrystalline silicon directly, for example, on a glass substrate having thereon a conductive film by plasma CVD (plasma chemical vapor deposition) has also been made.
Another method for solving the above problem is proposed in Japanese Patent Laid-Open No. 82997/1997. According to this method, amorphous silicon is crystallized by a metal catalyst to crystallize all crystalline layers of the same p- or n-conductivity type or all crystalline layers of the same p- or n-conductivity type including a BSF (back surface field) layer.
According to the conventional crystalline silicon thin film semiconductor device and crystalline silicon thin film photovoltaic device, however, when the amorphous silicon is crystallized on the glass substrate by laser annealing, a large number of substrates cannot be treated at a time without difficulties. This poses a problem of throughput. Specifically, in order to convert amorphous thin film silicon by melt crystallization to a polycrystalline layer having an even grain diameter, a method should be used which comprises forming amorphous thin film silicon by plasma CVD, thermally releasing hydrogen contained in the amorphous thin film silicon, and then performing laser annealing. Therefore, the production of products involves a lot of troubles and a lot of time which lead to increased cost.
On the other hand, the production process, wherein polycrystalline silicon is formed directly on glass substrates or the like by plasma CVD, has problems of quality, such as low crystallinity of the resultant polycrystalline silicon. In a pn structure and a pin structure which are generally adopted in solar cells, a p-conductivity-type or n-conductivity-type polycrystalline silicon thin film should be formed directly on a glass substrate having on its surface a conductive film. However, the polycrystalline silicon thin film, which has been formed directly on the glass substrate by plasma CVD, is known to have problems such as low crystallinity and short carrier life time. In particular, p-conductivity-type polycrystalline silicon thin films formed by plasma CVD involve problems of very low crystallinity and poor crystallographic orientation which are serious technical problems.
Further, according to the method disclosed in Japanese Patent Laid-Open No. 82997/1997, nickel silicide (alloy of silicon with nickel) is likely to be left at a junction with other conductivity type. Further, even when the residual nickel silicide has been removed by etching, defects are likely to occur. Therefore, recombination at the junction increases. This leads to a fear of characteristics of solar cell devices being significantly lowered.
Accordingly, it is an object of the invention to provide a crystalline silicon thin film semiconductor device, a crystalline silicon thin film photovoltaic device, and a process for producing a crystalline silicon thin film semiconductor device which can realize high crystallinity of polycrystalline silicon, excellent crystallographic orientation, high characteristics, and high productivity.
According to the first feature of the invention, a crystalline silicon thin film semiconductor device comprises:
a conductive substrate or a substrate having on its surface a conductive layer;
a crystallographically oriented first polycrystalline silicon layer which has been formed by introducing a metal catalyst element into an amorphous silicon layer, formed on the surface of the conductive substrate or the conductive layer, or so as to come into contact with the surface portion of the amorphous silicon layer, and heat treating the amorphous silicon layer to crystallize the amorphous silicon layer; and
a second polycrystalline silicon layer which has been formed, using the first polycrystalline silicon layer as a seed crystal, so as to have the same conductivity type as the first polycrystalline silicon layer.
According to this construction, a metal catalyst element is introduced into an amorphous silicon layer, provided on a substrate, or so as to come into contact with the amorphous silicon layer, followed by heat treatment to convert the amorphous silicon layer at a low temperature through the action of the metal catalyst element to a crystallographically oriented first polycrystalline silicon layer. When this first silicon layer is used as a seed crystal to form a second polycrystalline silicon layer on the surface of the first silicon layer, the resultant second polycrystalline silicon layer has the same crystallographic orientation as the first polycrystalline silicon layer as the substrate and high crystallinity. Likewise, the third polycrystalline silicon layer formed using the second polycrystalline silicon layer as a substrate has high crystallinity and is crystallographically oriented. As a result, a crystalline silicon thin film semiconductor device can be produced which can realize high crystallinity, excellent crystallographic orientation, high characteristics, and high productivity. Further, no silicide is left at a junction with other conductivity type. Therefore, there is no need to provide the step of removing silicide, and no defect attributable to silicide occurs.
According to the second feature of the invention, a crystalline silicon thin film photovoltaic device comprises:
a conductive substrate or an insulating substrate having on its surface a conductive layer;
a first polycrystalline silicon layer of a first conductivity type which has been formed by introducing a metal catalyst element into an amorphous silicon layer, formed on the surface of the conductive substrate or the conductive layer, or so as to come into contact with the surface portion of the amorphous silicon layer, and heat treating the amorphous silicon layer to crystallize the amorphous silicon layer;
a second polycrystalline silicon layer which has been formed, using the first polycrystalline silicon layer as a seed crystal, so as to have the same conductivity type as the first conductivity type;
a substantially i-type third polycrystalline silicon layer provided on the second polycrystalline silicon layer;
a fourth polycrystalline silicon layer that is provided on the third polycrystalline silicon layer and is of a second conductivity type which is different from the first conductivity type; and
an electrode part provided on the fourth polycrystalline silicon layer.
According to the third feature of the invention, a crystalline silicon thin film photovoltaic device comprises:
an insulating substrate having on its surface an electrode;
a first polycrystalline silicon layer of a first conductivity type which has been formed by introducing a metal catalyst element into an amorphous silicon layer, formed on the electrode of the insulating substrate, or so as to come into contact with the surface portion of the amorphous silicon layer, and heat treating the amorphous silicon layer to crystallize the amorphous silicon layer;
a second polycrystalline silicon layer which has been formed, using the first polycrystalline silicon layer as a seed crystal, so as to have the same conductivity type as the first conductivity type;
a third polycrystalline silicon layer which is provided on the second polycrystalline silicon layer and is of a second conductivity type which is different from the first conductivity type; and
an electrode part provided on the third polycrystalline silicon layer.
In the construction of the second and third features of the invention, a metal catalyst element is introduced into an amorphous silicon layer, provided on a substrate, or so as to come into contact with the amorphous silicon layer, followed by heat treatment to convert the amorphous silicon layer at a low temperature through the action of the metal catalyst element to a crystallographically oriented first polycrystalline silicon layer. When this first silicon layer is used as a seed crystal to form a second polycrystalline silicon layer on the surface of the first silicon layer, the resultant second polycrystalline silicon layer has the same crystallographic orientation as the first polycrystalline silicon layer as the substrate and high crystallinity. Likewise, the third polycrystalline silicon layer formed using the second polycrystalline silicon layer as a substrate has high crystallinity and is crystallographically oriented. Therefore, a crystalline silicon thin film photovoltaic device can be produced which can realize high crystallinity, crystallographic orientation, high characteristics, and excellent productivity.
According to the fourth feature of the invention, a process for producing a crystalline silicon thin film semiconductor device, comprises the steps of:
providing a conductive substrate or a substrate having on its surface a conductive layer and forming an amorphous silicon thin film on the surface of the conductive substrate or the surface of the conductive layer in the substrate;
introducing a metal catalyst element into the amorphous silicon layer or so as to come into contact with the surface portion of the amorphous silicon layer, and heat treating the amorphous silicon layer to crystallize the amorphous silicon layer and to form a crystallographically oriented first polycrystalline silicon layer;
forming, on the first polycrystalline silicon layer, a second polycrystalline silicon layer, of the same conductivity type as the first polycrystalline silicon layer, using the first polycrystalline silicon layer as a seed crystal; and
forming, on the second polycrystalline silicon layer, a third polycrystalline silicon layer of a second conductivity type which is different from the conductivity type of the second polycrystalline silicon layer.
According to this production process, an amorphous silicon thin film is formed on the surface of a substrate, and a metal catalyst element is introduced into the amorphous silicon layer or so as to come into contact with the surface portion of the amorphous silicon layer, followed by heat treatment of the amorphous silicon layer. This can crystallize the amorphous silicon layer at a low temperature to form a crystallographically oriented first polycrystalline silicon layer. When this first polycrystalline silicon layer is used as a seed crystal to form, on the first polycrystalline silicon layer, a second polycrystalline silicon layer of the same conductivity type as the first polycrystalline silicon layer, the second polycrystalline silicon layer has the same crystallographic orientation as the first polycrystalline silicon layer. Further, a third polycrystalline silicon layer, of the conductivity type which is different from that of the second polycrystalline silicon layer, is formed on the second polycrystalline silicon layer to constitute a semiconductor device having a pn structure. Therefore, a crystalline silicon thin film semiconductor device can be produced which can realize high crystallinity, crystallographic orientation, high characteristics, and excellent productivity.